The present invention relates to position calculating methods and devices.
A position calculating device using GPS (global positioning system) may be integrated into a mobile phone, a car navigation apparatus, or the like.
A position calculating device may calculate its own position by performing a predetermined position calculation using the position as an unknown quantity. This calculation may be based on satellite positions, satellite speeds, satellite movement directions, and pseudo distances from GPS satellites to the position calculating device. In addition, an error is included in the calculated pseudo distance due to bias of the clock provided in the position calculating device. Since it can be difficult to correct the clock bias, it is common to perform the position calculation under conditions in which the clock bias is also included as an unknown quantity.
Because such errors relate to the clock, there is not only the clock bias, but also an error (hereinafter, referred to as a “time error”) included in the time that is counted. Regarding the position of a satellite required when calculating the pseudo distance, the time that is counted serves as the reference. For this reason, when the position of a satellite deviates due to a time error, the pseudo distance becomes inaccurate and this affects the accuracy of the calculated position. Japanese Patent Application No. JP-T-2004-518135 discloses a method of calculating the position under conditions in which a time error is considered.
One known way to address the issue of time error is by synchronization with a received GPS satellite signal. This may be satisfactory when the receiving intensity of a satellite signal is sufficiently strong. However, when the position calculating device is located, for example, within a building, the intensity of a received signal may be weak. Accordingly, it may be difficult in such instances to acquire the accurate time information by synchronization with a GPS satellite signal. For this reason, in some cases it may be difficult to perform accurate position calculations because accurate time cannot be acquired.